A lubricating oil for an internal combustion engine, particularly for a diesel engine, typically contains a combination of a metal detergent and an ashless dispersant as a detergent dispersant. In general, examples of the metal detergent include sulfonate, phenate, salicylate and phosphonate of an alkali metal or an alkaline earth metal, and an overbased substance thereof.
A countermeasure against environmental pollution caused by nitrogen oxides (NOx) and particulate matters (PM) in exhaust gas has been of great interest in the diesel engine among the internal combustion engine. Accordingly, reduction in the nitrogen oxides and particulate matters in the exhaust gas has been urgent required. As the countermeasure, in order to reduce NOx, exhaust gas recirculation (EGR) is enhanced or a timing of a fuel injection is delayed to lower a combustion peak temperature. However, since the decrease in the combustion peak temperature causes increase in black exhaust and PM, an exhaust gas aftertreatment device needs to be attached. As the exhaust gas aftertreatment device, a PM trap, an oxidation catalyst or the like has been examined. However, since having a filter structure, the PM trap, the oxidation catalyst or the like is clogged with a metal content in a typical diesel engine oil.
On the other hand, reduction of the metal content in the oil, in other words, reduction of a metal detergent and an antiwear agent causes deterioration of detergency and wear resistance. Particularly, the reduction of the metal detergent results in decrease in an initial base value. Accordingly, in order to maintain long-drain capabilities at the same level as in a typical oil, development of a new lubricating oil for an internal combustion engine has been desired.
For instance, Patent Literature 1 discloses formulation of an additive containing amino alcohol and a reduced content of a metal detergent. Patent Literature 1 discloses that, even when the content of the metal detergent is small (i.e., a low ash content), detergency within the diesel engine can be enhanced by blending this additive in the lubricating oil, thereby prolonging a lifetime of the lubricating oil.